The present invention generally pertains to injection molding and is particularly directed to control of hold pressure in a multi-parting injection molding system.
Hold pressure is the pressure applied to plastic material injected into a cooling cavity after the cooling cavity is initially filled by an injection unit. Hold pressure is applied to force additional plastic material into the cooling cavity as the initially injected material shrinks in the mold upon cooling in order to assure that the plastic material completely fills the cavity. Otherwise the molded product may be crimped.
In the prior art, the injection unit is used to apply hold pressure. However, when using a multi-parting molding system, the use of the molding unit to apply hold pressure may increase the cycle time. Also, with typical stack mold systems, when the stack mold is opened and the injection unit is being recharged, the injection unit is separated from the remainder of the feed system that carries the plastic material to the cooling cavities, thereby reducing the time available for the injection unit to apply pressure.